This invention relates to the improvement of silicon carbide whisker reinforced alumina based ceramic composites and method for making same. Ceramics is one of the most promising materials to be used in high-technology industries. Owing to its excellent properties of mechanical strength, hardness, high temperature stability as well as corrosion resistance, it has a great potential in the applications in high-temperature environments. However brittleness and low reliability are the two main problems limiting its use. Therefore, a large portion of current research work is aimed at making tougher ceramic materials. The experiences in making metallic composites and fiber reinforced photic may be followed in manufacturing of ceramic matrix composites. The evidences have shown that this method can effectively and significantly improve the toughness of ceramic materials and, thus, reduce the occurances of catastraphic failures.
Generally speaking, ceramic composites may be categorized according to the reinforcements being used: fibers or particulates. In early years, polycrystalline silicon carbide fibers are in corporated into the ceramic matrix. The material is strengthened and toughened through the mechanisms of crack deflection and fiber pullout. However, the length fibers cause fabricating process difficult and the oriented microstructure consequently will lead to the anisotropy of mechanical strength in the material. Moreover, S:C, owing to the metallic impurities devived from fabricating process begins to soften at temperatures as low as 900.degree. C. and losing its strengthening effect. Therefore, polycrystalline S:C fibers are gradually replaced by single crystal S:C whiskers. The advantages of using whiskers are that is makes composite fabrication easire, and that the composite babricated has high-temperature resistance. The main drawback of S:C whiskers (or fibers) reinforced ceramics is originated in the low thermal expansion coefficient of S:C which is, for instance, about half of that of alumina. This would generate residual tensile stress in the composite and damage its mechanical strength. Especially if the amount of S:C whiskers added is more than 30-40 vol. %, the strength and toughness would be adversely affected. This phenomenon has been widely reported in relevant literatures.
Adding heterogeneous ceramic granules or particles (mostly carbides, such as titanium carbide and boron carbide) in ceramic matrix, will also reinforce the composite according to the toughening principles of crack deflection and crack bridging. Addition of carbide particles is convenient for fabrication steps. However, the toughening effect is not so significant as with the addition of fibers.
Both methods described above for the manufacture of ceramic composites require very high sintering-temperature to gain full densed material in order to develope the desired mechanical properties consequently. This would somewhat limit the attempt of making use of this kind of material because of its high production cost.
Several inventions in S:C whiskers reinforced ceramic composites are found in U.S. Pat. Nos. 4,634,608 and 4,657,877, and Japanese Patents No. 61-270266, No. 61-286271 and No. 62-235266, whose raw materials consisting:
(a) 10-40 vol. % S:C whiskers. PA1 (b) 7-35 wt. % Zirconia (ZrO.sub.2) PA1 (c) 0.7-7 wt. % sintering additives for alumina, including 0.7 to 7 wt. % at least one of the following oxides: calcium oxide, magnesia, silica dioxide, nickel oxide, yttrium oxide or lanthanum oxide. PA1 (d) 0.05 to 5 wt. % sintering additives for silicon carbide, including boron, carbon, aluminum nitride, boron carbide and borides of silicon, aluminum and nitrogen. PA1 (e) 5-30% carbides, borides, nitrides or oxides of group IVA, VA, VIA elements in the periodic table. PA1 (f) Alumina constituting the balance.
Although the ceramic composites described in above-mentioned pattents are better than single-phase ceramics in some aspects of mechanical properties, drawbacks still remain as listed in the following:
(1) As disclosed in U.S. Pat. No. 4,657,877 and Japanese Patent 62-265182, zirconia is incorporated into a ceramix matrix as a toughening agent. Zirconia particles can transform from cubic to monoclinic crystal symmetry and because of its volumetric expansion, compressive stress formed in the tip region of cracks, and break the cracks into many microcrack. Phase transformation and microcracking both dissipate the energy for crack propagation, and hence gain the toughening effect. However, in the high-temperature conditions (higher than 1,100.degree. C.) cubic zirconia becomes stable and no phase transformation will take place in the ceramic composites. Therefore, no toughening effect will be observed. This material is not to be used higher in temperatures than 1,100.degree. C. Besides, the addition of zirconia will cause the hardness of the composites decline.
(2) As disclosed in Japanese Patent No. 62-235266, the composition of the invented composites therein consists of (a), (d), (e) and (f). To gain densification of alumina matrix and good mechanically interfacial bonding the sintering temperature has to be raised to higher than 1,650.degree. C.
(3) As disclosed in Japanese Patent No. 62-235266 and No. 62-41776, the composition of the ceramic composite consists of (a), (c), (d), (e) and (f). A sintering additive for alumina is added. The sintering temperature of the composition is still very high in the range of 1,650.degree.-1,850.degree. C., for acquiring full densification of the composites. The added component does not show any toughening effect. Otherwise, it will reduce the toughness because of inhomogeneous material.